我试图验证用于在MySQL 5.5中执行多表更新的正确语法。我有两个表，第一个是这样的结构：
| *post_id* | lat | long |
其中post_id是唯一标识符。第二个表是Wordpress postmeta表：
| *meta_id* | post_id | meta_key | meta_value |
其中meta_id是唯一标识符。第一个表中的Post_ID匹配第二个表。第二个表格中的部分meta_keys包括list-lat和list-long。
我希望使用第二个表中long为0.000000且long不为0.000000的第二个表中的相应值更新第一个表中lat的值。以下select语句标识130行：
SELECT * FROM bch_postmeta
JOIN bch_coords ON bch_coords.post_id = bch_postmeta.post_id
WHERE bch_coords.long = 0.000000
AND bch_coords.lat <> 0.000000 AND bch_postmeta.meta_key = 'list-long'
这是预期的。如果我尝试一个简单的UPDATE并模拟它，我再次影响130行：
UPDATE bch_coords
SET bch_coords.long = 0.0
WHERE bch_coords.long = 0.000000 AND bch_coords.lat <> 0.000000
但是，一旦我尝试合并第二个表，查询就不再起作用了。以下两个查询都会影响0行。第二个当然是我希望进行的更新。
UPDATE bch_coords, bch_postmeta
SET bch_coords.long = 0.0
WHERE bch_coords.long = 0.000000 AND bch_coords.lat <> 0.000000
UPDATE bch_coords, bch_postmeta
SET bch_coords.long = bch_postmeta.meta_value
WHERE bch_coords.post_id = bch_postmeta.post_id AND bch_postmeta.meta_key = 'list-long'
AND bch_coords.long = 0.000000 AND bch_coords.lat <> 0.000000
我没有使用ANSI联接，因为由于某种原因它们在UPDATE中使用时导致查询失败（此时也是个谜）。
有人能说清楚我在这里做错了什么吗？
我在下面创建了一个SQLFiddle降价：
MySQL 5.5架构设置：
CREATE TABLE bch_coords
(`post_id` int, `lat` numeric, `long` numeric, UNIQUE(post_id))
;
INSERT INTO bch_coords
(`post_id`, `lat`, `long`)
VALUES
(1, 15.986132, 136.82515),
(2, 0.000000, 0.000000),
(3, -15.23850, 0.000000),
(4, 136.32067, 0.000000),
(5, -87.123567, 56.12396)
;
CREATE TABLE bch_postmeta
(`meta_id` int, `post_id` int, `meta_key` varchar(13), `meta_value` varchar(15), UNIQUE(meta_id))
;
INSERT INTO bch_postmeta
(`meta_id`, `post_id`, `meta_key`, `meta_value`)
VALUES
(1, 1, '''list-lat''', '15.986132'),
(2, 1, '''list-long''', '136.82515'),
(3, 1, '''country''', '''Switzerland'''),
(4, 1, '''state''', '''Valais'''),
(5, 2, '''list-lat''', '0.000000'),
(6, 2, '''list-long''', '0.000000'),
(7, 2, '''country''', '''Australia'''),
(8, 3, '''list-lat''', '-15.00'),
(9, 3, '''list-long''', '173.62198'),
(10, 3, '''country''', '''USA'''),
(11, 4, '''list-lat''', '136.32067'),
(12, 4, '''list-long''', '5.123997'),
(13, 5, '''list-lat''', '-88.125'),
(14, 5, '''list-long''', '56.12396')
;
所以，我有一个看起来像这样的数据集......
SELECT * FROM bch_coords;
+---------+------+------+
| post_id | lat | long |
+---------+------+------+
| 1 | 16 | 137 |
| 2 | 0 | 0 |
| 3 | -15 | 0 |
| 4 | 136 | 0 |
| 5 | -87 | 56 |
+---------+------+------+
SELECT * FROM bch_postmeta;
+---------+---------+-------------+---------------+
| meta_id | post_id | meta_key | meta_value |
+---------+---------+-------------+---------------+
| 1 | 1 | 'list-lat' | 15.986132 |
| 2 | 1 | 'list-long' | 136.82515 |
| 3 | 1 | 'country' | 'Switzerland' |
| 4 | 1 | 'state' | 'Valais' |
| 5 | 2 | 'list-lat' | 0.000000 |
| 6 | 2 | 'list-long' | 0.000000 |
| 7 | 2 | 'country' | 'Australia' |
| 8 | 3 | 'list-lat' | -15.00 |
| 9 | 3 | 'list-long' | 173.62198 |
| 10 | 3 | 'country' | 'USA' |
| 11 | 4 | 'list-lat' | 136.32067 |
| 12 | 4 | 'list-long' | 5.123997 |
| 13 | 5 | 'list-lat' | -88.125 |
| 14 | 5 | 'list-long' | 56.12396 |
+---------+---------+-------------+---------------+
...并且在更新之后，我希望有一个看起来像这样的数据集......
SELECT * FROM bch_coords;
+---------+------+------+
| post_id | lat | long |
+---------+------+------+
| 1 | 16 | 137 |
| 2 | 0 | 0 |
| 3 | -15 | 173 |
| 4 | 136 | 5.1 |
| 5 | -87 | 56 |
+---------+------+------+
答案 0 :(得分：0)
试试这个：
UPDATE bch_coords
SET bch_coords.long = (SELECT bch_postmeta.meta_value
FROM bch_postmeta
WHERE bch_postmeta.post_id = bch_coords.post_id
AND bch_postmeta.meta_key = 'list-long'
LIMIT 1)
WHERE bch_coords.long = 0.000000
AND bch_coords.lat <> 0.000000
或者这个：
UPDATE bch_coords
INNER JOIN bch_postmeta ON (bch_coords.post_id = bch_postmeta.post_id AND bch_postmeta.meta_key = 'list-long')
SET bch_coords.long = bch_postmeta.meta_value
WHERE bch_coords.long = 0.000000
AND bch_coords.lat <> 0.000000